A sheet storage cassette includes a cassette base and inner rails. The cassette base has side plates facing each other parallel to insertion/withdrawal directions of the sheet storage cassette. The inner rails each have positioning bosses that protrude from each of different positions on the inner rails in a longitudinal direction thereof and a hook portion that protrudes in an L-shape in cross section toward the cassette base and is formed between the positioning bosses. On the side plates of the cassette base, there are provided mounting holes that receive the positioning bosses fitted thereinto at the time of mounting the cassette base to the inner rails, positioning surfaces that comes into contact with two locations on each of the inner rails in a longitudinal direction thereof, and fitted portions that are fitted together with the hook portion in a direction orthogonal to the insertion/withdrawal directions.